Deep X-ray lithography involves a substrate which is covered by a thick photoresist, typically several hundred microns in thickness, which is exposed through a mask by X-rays. X-ray photons are much more energetic than optical photons, which makes complete exposure of thick photoresist films feasible and practical. Furthermore, since X-ray photons are short wavelength particles, diffraction effects which typically limit device dimensions to two or three wavelengths of the exposing radiation are absent for mask dimensions above 0.1 micron. If one adds to this the fact that X-ray photons are absorbed by atomic processes, standing wave problems, which typically limit exposures of thick photoresists by optical means, become a nonissue for X-ray exposures. The use of a synchrotron for the X-ray sources yields high flux densities--several watts per square centimeter--combined with excellent collimation to produce thick photoresist exposures without any horizontal runout. Locally exposed patterns should therefore produce vertical photoresist walls if a developing system with very high selectivity between exposed and unexposed photoresist is available. This requirement has been satisfied using polymethylmethacrylate (PMMA) as the X-ray photoresist, and an aqueous developing system. See, H. Guckel, et al., "Deep X-ray and UV Lithographies for Micromechanics," Technical Digest, Solid State Sensor and Actuator Workshop, Hilton Head, S.C., Jun. 4-7, 1990, pp. 118-122.
Deep X-ray lithography may be combined with electroplating to form high aspect ratio structures. To do so requires that the substrate be furnished with a suitable plating base prior to photoresist application. Commonly, this involves a sputtered film of adhesive metal, such as chromium or titanium, which is followed by a thin film of metal which is suitable for electroplating the metal to be plated. In appropriate cases, the use of an initial layer of adhesive metal is not necessary. Exposure through a suitable mask and development are followed by electroplating. This process results, after cleanup, in fully attached metal structures with very high aspect ratios. Such structures were reported by W. Ehrfeld and co-workers at the Institute for Nuclear Physics (KFK) at Karlsruhe in West Germany. Ehrfeld termed the process "LIGA" based on the first letters for the German words for lithography and electroplating. A general review of the LIGA process is given in the article by W. Ehrfeld, et al., "LIGA Process: Sensor Construction Techniques Via X-Ray Lithography," Technical Digest IEEE Solid State Sensor and, Actuator Workshop, 1988, pp. 1-4.
A crucial factor in the production of microminiature devices, such as those formed by the LIGA process, is the photoresist that is used. As noted, PMMA has been successfully used as the photoresist for formation of LIGA structures. The PMMA films have been produced by casting of liquid MMA directly on the substrate, with the film being reduced to the desired thickness, generally not more than two to three hundred microns, with a casting jig. The cast film is then solidified, typically by utilizing a polymerization agent or initiator in the casting solution and a cross-linking agent which results in cross-linking upon curing. There are several disadvantages and limitations of the PMMA films formed in this manner. The casting procedures require special equipment and fixturing, which adds to the time and cost of the process. As with almost all casting operations, a heat cycle is necessary to produce the solidified film. Typically, annealing cycles up to 110.degree. C. are required. These heat cycles build up strain in the film due to a significant mismatch in thermal expansion coefficients between the PMMA photoresist and the substrate. Internal strain in the photoresist also occurs due to the shrinking of the film during curing, which has been observed to result in up to a 20% shrinkage of the film from its as-cast state. As a consequence, the cast film, after curing, often has poor adhesion to the substrate and can buckle off the substrate. Even where adhesion to the substrate is retained, the internal strain that is built into the film can result in distortion of the walls formed in the film after patterning of the photoresist by X-ray exposure and development.
PMMA photoresist films are typically cross-linked through the addition of a cross-linking agent to the casting solutions to minimize crazing of the films. Because of this cross-linking, it is necessary to have an additional X-ray exposure step, a blanket exposure of the entire photoresist film, followed by development of the film to remove the resist when its use is complete. Even with the use of cross-linking agents, the maximum thickness of resists which have been successfully cast, exposed, and developed have been in the range of about 300 microns. Most samples of PMMA photoresist films having thickness greater than 200 microns have unacceptable amounts of crazing and adhesion loss. Typically, the cast PMMA films may only be used once because such films are found to exhibit significant crazing after the microplating of metal into the patterned openings in the film during the electroplating step of the LIGA process.
The thicknesses of photoresist utilized for micromechanical processing is typically a few hundred microns or less, which is below the typical thicknesses of preformed photoresist sheets. Ehrfeld and co-workers have reported attempts to adhere a preformed PMMA photoresist sheet to a substrate, with the photoresist being calendered before adherence to the substrate to reduce the sheet to the desired thickness for carrying out the LIGA process. However, such attempts were reported to be unsuccessful, apparently because the strain fields in the calendered photoresist were excessive.